linux-kernel - Re: [PATCH] KVM: x86/mmu: Implement memslot deletion for TDX

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Message-ID: <ZnTEhQo2r3Uz9rDY@google.com>
Date: Thu, 20 Jun 2024 17:08:37 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc: yan.y.zhao@...el.com, dmatlack@...gle.com, erdemaktas@...gle.com, 
	isaku.yamahata@...el.com, kai.huang@...el.com, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, pbonzini@...hat.com, sagis@...gle.com
Subject: Re: [PATCH] KVM: x86/mmu: Implement memslot deletion for TDX

On Thu, Jun 20, 2024, Rick Edgecombe wrote:
> Force TDX VMs to use the KVM_X86_QUIRK_SLOT_ZAP_ALL behavior.
> 
> TDs cannot use the fast zapping operation to implement memslot deletion for
> a couple reasons:
> 1. KVM cannot zap TDX private PTEs and re-fault them without coordinating

Uber nit, this isn't strictly true, for KVM's definition of "zap" (which is fuzzy
and overloaded).  KVM _could_ zap and re-fault *leaf* PTEs, e.g. BLOCK+UNBLOCK.
It's specifically the full teardown and rebuild of the "fast zap" that doesn't
play nice, as the non-leaf S-EPT entries *must* be preserved due to how the TDX
module does is refcounting.